CRYSTAL SIZE DISTRIBUTION ANALYSIS OF QUATERNARY LAVA FLOWS FROM MAKUSHIN VOLCANO, ALASKA

Mt. Makushin is an active stratovolcano on Unalaska Island in the eastern Aleutian arc. The volcano has been active since the Pleistocene, erupting basaltic to andesitic lavas; however, most studies have focused on its Holocene record. We present a crystal size distribution (CSD) analysis of plagioclase crystals for six samples collected along the northeast flank of the volcano in order to explore the complex magmatic system of the older probable Pleistocene lavas. The samples, primarily basalts and basaltic andesites, are dominated by plagioclase crystals and contain minor amounts of coarse clinopyroxene. The CSD curves display steeper slopes (ranging from -43 to -21 ln(n)/L) at smaller crystal sizes and shallower slopes (-4.5 to 1.7 ln(n)/L) at larger sizes, suggestive of two crystal size populations. The break in slope occurs at crystal lengths between 0.1 and 0.15 cm. In several samples, large plagioclase glomerocrysts are a conspicuous feature and exhibit disequilibrium textures. Differences in size and composition indicate that these glomerocrysts represent distinct crystal subpopulations.

The two-population trend seen in the CSD data could be due to magma mixing, inherited crystals, or entrained crystals captured as magma rose to the surface. These observations are consistent with interpretations based upon chemical and mineral composition data (Nye et al., 1986) suggesting that the lavas were erupted from a regularly refilled chamber until the early Holocene, when a magma flux caused short-lived high-volume eruptions. Either the replenishment of a chamber or the mixing of magma bodies could have caused the two-population trend seen in the CSD data. The similarity of the crystal sizes marking the break in slope in the CSD curve for each sample suggests that the smaller populations of crystals in each flow experienced comparable magma chamber residence times. ⁴⁰Ar/³⁹Ar dating of the basalt samples (in progress) will further restrain residence times for the magmas and refine interpretations of the magmatic system.